Our been-there-done-that Google Cardboard tips
You want strong, thin cardboard: try this with anything flimsy and you’ll end up with a ball of mashed paper and tears.
When you print the plans, make sure you print at 100% – a lot of printer software tends to helpfully downsize your printout to 90% to fit it all in one page with a nice spacious border.
A good sharp scalpel and a ruler (to guide your blade) are essential – if you use scissors, as Google cheerily suggests, it’ll take four hours, and the end result will look like a wasps’ nest.
When it doesn’t fit together properly, just keep taping it together until it looks about right.
If all else fails, you can achieve a similar effect by holding the two lenses roughly 40mm away from your phone and moving your face in front of them.